### Feb 2021, Volume 16 Issue 1

Cover illustration

• Zeeman deceleration is widely used for cooling of atoms and molecules. However, further cooling of molecules to ultracold temperature following Zeeman deceleration is always hindered by the low density of the decelerated molecular packet provided by the traditional Zeeman decelerator. Here the authors propose an experimentally viable scheme, which employs a moving magnetic trap to bring a large density of lithium atom and methyl radical into standstill, enabling cold collisio [Detail] ...

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• RESEARCH HIGHLIGHT
Zigao Dai

Self-organized criticality during the prompt phase of gamma-ray bursts shows that they may arise from magnetic reconnection events.

• RESEARCH ARTICLE
Fen Lyu, Ya-Ping Li, Shu-Jin Hou, Jun-Jie Wei, Jin-Jun Geng, Xue-Feng Wu

The variability in multi-pulse gamma-ray bursts (GRBs) may help to reveal the mechanism of underlying processes from the central engine. To investigate whether the self-organized criticality (SOC) phenomena exist in the prompt phase of GRBs, we statistically study the properties of GRBs with more than 3 pulses in each burst by fitting the distributions of several observed physical variables with a Markov Chain Monte Carlo approach, including the isotropic energy Eiso, the duration time T, and the peak count rate P of each pulse. Our sample consists of 454 pulses in 93 GRBs observed by the CGRO/BATSE satellite. The best-fitting values and uncertainties for these power-law indices of the differential frequency distributions are: $αE d=1.54 ±0.09$, $αT d= 1.82−0.15+0.14$ and $αPd=2.09−0.19+0.18$, while the power-law indices in the cumulative frequency distributions are: $αE c= 1.44−0.10+0.08$, $αTc=1.75−0.13+0.11$ and $αP c= 1.99−0.19+0.16$. We find that these distributions are roughly consistent with the physical framework of a Fractal-Diffusive, Self-Organized Criticality (FD-SOC) system with the spatial dimension S = 3 and the classical diffusion β=1. Our results support that the jet responsible for the GRBs should be magnetically dominated and magnetic instabilities (e.g., kink model, or tearing-model instability) lead the GRB emission region into the SOC state.

• VIEW & PERSPECTIVE
Qichun Zhang

Carbon-rich materials are the key.

• RESEARCH HIGHLIGHT
Shuai Dong

Ferroelectrics in the two-dimensional limit is crucial for further miniaturization of micro-electro-mechanical systems. CuInP2S6, a star of new developing vdW ferroelectrics, will answer some long-standing questions.

• RESEARCH ARTICLE
Qinye Li, Siyao Qiu, Baohua Jia

Water electrolysis is to split water into hydrogen and oxygen using electricity as the driving force. To obtain low-cost hydrogen in a large scale, it is critical to develop electrocatalysts based on earth abundant elements with a high efficiency. This computational work started with Cobalt on CoTa2O6 surface as the active site, CoTa2O6/Graphene heterojunctions have been explored as potential oxygen evolution reaction (OER) catalysts through density functional theory (DFT). We demonstrated that the electron transfer (δ) from CoTa2O6 to graphene substrate can be utilized to boost the reactivity of Co-site, leading to an OER overpotential as low as 0.30 V when N-doped graphene is employed. Our findings offer novel design of heterojunctions as high performance OER catalysts.

• RESEARCH ARTICLE
Sadegh Imani Yengejeh, William Wen, Yun Wang

Transition metal dichalcogenide (TMD) monolayers attract great attention due to their specific structural, electronic and mechanical properties. The formation of their lateral heterostructures allows a new degree of flexibility in engineering electronic and optoelectronic dervices. However, the mechanical properties of the lateral heterostructures are rarely investigated. In this study, a comparative investigation on the mechanical characteristics of 1H, 1T′ and 1H/1T′ heterostructure phases of different TMD monolayers including molybdenum disulfide (MoS2) molybdenum diselenide (MoSe2), Tungsten disulfide (WS2), and Tungsten diselenide (WSe2) was conducted by means of density functional theory (DFT) calculations. Our results indicate that the impact of the lateral heterostructures has a relatively weak mechanical strength for all the TMD monolayers. The significant correlation between the mechanical properties of the TMD monolayers and their structural phases can be used to tune their stiffness of the materials. Our findings, therefore, suggest a novel strategy to manipulate the mechanical characteristics of TMDs by engineering their structural phases for their practical applications.

• RESEARCH ARTICLE
Yuan-Yuan Wang, Feng-Ping Li, Wei Wei, Bai-Biao Huang, Ying Dai

Van der Waals (vdW) heterobilayers formed by two-dimensional (2D) transition metal dichalcogenides (TMDCs) created a promising platform for various electronic and optical properties. ab initio band results indicate that the band offset of type-II band alignment in TMDCs vdW heterobilayer could be tuned by introducing Janus WSSe monolayer, instead of an external electric field. On the basis of symmetry analysis, the allowed interlayer hopping channels of TMDCs vdW heterobilayer were determined, and a four-level k·p model was developed to obtain the interlayer hopping. Results indicate that the interlayer coupling strength could be tuned by interlayer electric polarization featured by various band offsets. Moreover, the difference in the formation mechanism of interlayer valley excitons in different TMDCs vdW heterobilayers with various interlayer hopping strength was also clarified.

• TOPICAL REVIEW
Ming-Chao Xiao, Jie Liu, Yuan-Yuan Hu, Shuai Wang, Lang Jiang

With the development of device engineering and molecular design, organic field effect transistors (OFETs) with high mobility over 10 cm2·V−1·s−1 have been reported. However, the nonideal doubleslope effect has been frequently observed in some of these OFETs, which makes it difficult to extract the intrinsic mobility OFETs accurately, impeding the further application of them. In this review, the origin of the nonideal double-slope effect has been discussed thoroughly, with affecting factors such as contact resistance, charge trapping, disorder effects and coulombic interactions considered. According to these discussions and the understanding of the mechanism behind double-slope effect, several strategies have been proposed to realize ideal OFETs, such as doping, molecular engineering, charge trapping reduction, and contact engineering. After that, some novel devices based on the nonideal double-slope behaviors have been also introduced.

• TOPICAL REVIEW
Si Liu, Hongnan Wu, Xiaotao Zhang, Wenping Hu

Rubrene, a superstar in organic semiconductors, has achieved unprecedented achievements in the application of electronic devices, and research based on its various photoelectric properties is still in progress. In this review, we introduced the preparation of rubrene crystal, summarized the applications in organic optoelectronic devices with the latest research achievements based on rubrene semiconductors. An outlook of future research directions and challenges of rubrene semiconductor for applications is also provided.

• TOPICAL REVIEW
Miao Zhu, Muhammad Umair Ali, Changwei Zou, Wei Xie, Songquan Li, Hong Meng

Tactile and temperature sensors are the key components for e-skin fabrication. Organic transistors, a kind of intrinsic logic devices with diverse internal configurations, offer a wide range of options for sensor design and have played a vital role in the fabrication of e-skin-oriented tactile and temperature sensors. This research field has attained tremendous advancements, both in terms of materials design and device architecture, thereby leading to excellent performance of resulting tactile/temperature sensors. Herein, a systematic review of organic transistor-based tactile and temperature sensors is presented to summarize the latest progress in these devices. Particularly, we focus on spotlighting various device structures, underlying mechanisms and their performance. Lastly, an outlook for the future development of these devices is briefly discussed. We anticipate that this review will provide a quick overview of such a rapidly emerging research direction and attract more dedicated efforts for the development of next-generation sensing devices towards e-skin fabrication.

• REVIEW ARTICLE
Shuang Zhou, Lu You, Hailin Zhou, Yong Pu, Zhigang Gui, Junling Wang

Copper indium thiophosphate, CuInP2S6, has attracted much attention in recent years due to its van der Waals layered structure and robust ferroelectricity at room temperature. In this review, we aim to give an overview of the various properties of CuInP2S6, covering structural, ferroelectric, dielectric, piezoelectric and transport properties, as well as its potential applications. We also highlight the remaining questions and possible research directions related to this fascinating material and other compounds of the same family.

• TOPICAL REVIEW
Lin Ju, Mei Bie, Xiwei Zhang, Xiangming Chen, Liangzhi Kou

Two-dimensional Janus van der Waals (vdW) heterojunctions, referring to the junction containing at least one Janus material, are found to exhibit tuneable electronic structures, wide light adsorption spectra, controllable contact resistance, and sufficient redox potential due to the intrinsic polarization and unique interlayer coupling. These novel structures and properties are promising for the potential applications in electronics and energy conversion devices. To provide a comprehensive picture about the research progress and guide the following investigations, here we summarize their fundamental properties of different types of two-dimensional Janus vdW heterostructures including electronic structure, interface contact and optical properties, and discuss the potential applications in electronics and energy conversion devices. The further challenges and possible research directions of the novel heterojunctions are discussed at the end of this review.

• RESEARCH ARTICLE
Yang Liu, Le Luo

Chemistry in the ultracold regime enables fully quantum-controlled interactions between atoms and molecules, leading to the discovery of the hidden mechanisms in chemical reactions which are usually curtained by thermal averaging in the high temperature. Recently a couple of diatomic molecules have been cooled to ultracold regime based on laser cooling techniques, but the chemistry associated with these simple molecules is highly limited. In comparison, free radicals play a major role in many important chemical reactions, but yet to be cooled to submillikelvin temperature. Here we propose a novel method of decelerating CH3, the simplest polyatomic free radical, with lithium atoms simultaneously by travelling wave magnetic decelerator. This scheme paves the way towards co-trapping CH3 and lithium, so that sympathetical cooling can be used to preparing ultracold free radical sample.

• RESEARCH ARTICLE
Lu Qi, Guo-Li Wang, Shutian Liu, Shou Zhang, Hong-Fu Wang

We propose a scheme to investigate the topological phase transition and the topological state transfer based on the small optomechanical lattice under the realistic parameters regime. We find that the optomechanical lattice can be equivalent to a topologically nontrivial Su–Schrieffer–Heeger (SSH) model via designing the effective optomechanical coupling. Especially, the optomechanical lattice experiences the phase transition between topologically nontrivial SSH phase and topologically trivial SSH phase by controlling the decay of the cavity field and the optomechanical coupling. We stress that the topological phase transition is mainly induced by the decay of the cavity field, which is counter-intuitive since the dissipation is usually detrimental to the system. Also, we investigate the photonic state transfer between the two cavity fields via the topologically protected edge channel based on the small optomechanical lattice. We find that the quantum state transfer assisted by the topological zero energy mode can be achieved via implying the external lasers with the periodical driving amplitudes into the cavity fields. Our scheme provides the fundamental and the insightful explanations towards the mapping of the photonic topological insulator based on the micro-nano optomechanical quantum optical platform.

• RESEARCH ARTICLE
San-Dan Wang, Jin-Peng Yuan, Li-Rong Wang, Lian-Tuan Xiao, Suo-Tang Jia

We experimentally demonstrate the cesium electric quadrupole transition from the 6S1/2 ground state to the 7D3/2,5/2 excited state through a virtual level by using a single laser at 767 nm. The excited state energy level population is characterized by varying the laser power, the temperature of the vapor, and the polarization combinations of the laser beams. The optimized experimental parameters are obtained for a high resolution transition interval identification. The magnetic dipole coupling constant A and electric quadrupole coupling constant B for the 7D3/2,5/2 states are precisely determined by using the hyperfine levels intervals. The results, A = 7.39 (0.06) MHz, B = −0.19 (0.18) MHz for the 7D3/2 state, and A = −1.79 (0.05) MHz, B =1.05 (0.29) MHz for the 7D5/2 state, are in good agreement with the previous reported results. This work is beneficial for the determination of atomic structure information and parity non-conservation, which paves the way for the field of precision measurements and atomic physics.

• RESEARCH ARTICLE
Cheng-Bing Qin, Xi-Long Liang, Shuang-Ping Han, Guo-Feng Zhang, Rui-Yun Chen, Jian-Yong Hu, Lian-Tuan Xiao, Suo-Tang Jia

Monolayer transition metal dichalcogenides have emerged as promising materials for optoelectronic and nanophotonic devices. However, the low photoluminescence (PL) quantum yield (QY) hinders their various potential applications. Here we engineer and enhance the PL intensity of monolayer WS2 by femtosecond laser irradiation. More than two orders of magnitude enhancement of PL intensity as compared to the as-prepared sample is determined. Furthermore, the engineering time is shortened by three orders of magnitude as compared to the improvement of PL intensity by continuous-wave laser irradiation. Based on the evolution of PL spectra, we attribute the giant PL enhancement to the conversion from trion emission to exciton, as well as the improvement of the QY when exciton and trion are localized to the new-formed defects. We have created microstructures on the monolayer WS2 based on the enhancement of PL intensity, where the engineered structures can be stably stored for more than three years. This flexible approach with the feature of excellent long-term storage stability is promising for applications in information storage, display technology, and optoelectronic devices.

• RESEARCH ARTICLE
Yu-Fei Yan, Lan Zhou, Wei Zhong, Yu-Bo Sheng

Measurement-device-independent quantum key distribution (MDI-QKD) provides us a powerful approach to resist all attacks at detection side. Besides the unconditional security, people also seek for high key generation rate, but MDI-QKD has relatively low key generation rate. In this paper, we provide an efficient approach to increase the key generation rate of MDI-QKD by adopting multiple degrees of freedom (DOFs) of single photons to generate keys. Compared with other high-dimension MDI-QKD protocols encoding in one DOF, our protocol is more flexible, for our protocol generating keys in independent subsystems and the detection failure or error in a DOF not affecting the information encoding in other DOFs. Based on above features, our MDI-QKD protocol may have potential application in future quantum communication field.